This invention relates generally to electrical circuit breakers and more particularly to aircraft circuit breakers for interrupting electrical circuits on the occurrence of arcing conditions as well as current overload conditions in the circuits.
Thermally responsive electrical circuit breakers typically interrupt electrical circuits in response to the occurrence of selected overload conditions in the circuits to protect other equipment and wiring in the circuits from damage due to overheating, overcurrent or the like. Examples of such devices are shown in U.S. Pat. Nos. 3,361,882 and 4,400,677, the disclosures of which are incorporated herein by this reference. In these patents a control mechanism manually moves movable contacts into and out of engagement with complementary stationary contacts to open and close a circuit and a thermally responsive bimetallic member is operable to open the circuit in response to the occurrence of a selected overload current in the circuit. The bimetallic member is formed of metal materials having substantial electrical resistance properties and the member is disposed in the circuit breaker so that the member is self-heated and flexes to a selected extent to trip the mechanism to open the circuit breaker when selected overload current flows in the circuit for a selected period of time. The circuit breaker is adapted to be latched in the open circuit position until manually reset after it has been manually opened or has been opened in response to the occurrence of an overload current. It is also xe2x80x9ctrip freexe2x80x9d in that the circuit breaker will open in response to the occurrence of an overload current even if the manual resetting means is manually held in the circuit closing position.
Circuit breakers of this type are not designed to respond to low current events; however, there are certain low current events, viz, arcs, which are capable of causing fires or catastrophic events. For example, an electrical connection that is starting to become loose can result in sufficient energy to cause arcing yet have insufficient current drawn through the circuit to actuate the circuit breaker. Although household circuit breakers containing arc sensing technology are known, these devices are limited in their capabilities and are far too large to use in aircraft.
An object of the present invention is to provide an arc fault protection circuit breaker amenable to aircraft usage. Another object of the invention is the provision of a conventional aircraft circuit breaker which trips open upon sensing an overload current condition which is adapted to also provide protection from arcing conditions. Another object of the invention is to provide a circuit breaker which will open a circuit upon the occurrence of an arc fault as well as a thermal overcurrent condition and which provides trip indication to distinguish between a trip caused by an arc fault condition and one caused by a thermal or overcurrent condition.
Briefly stated, a circuit breaker made in accordance with the invention comprises movable contact means, complimentary stationary contact means and a control mechanism for normally holding the movable contact means in engagement with the stationary contact means in a closed circuit position. As in the above referenced patents, the control mechanism includes a thermally responsive bimetallic member having substantial electrical resistance properties which is disposed in the breaker circuit so that an overload current flowing in the circuit for a selected period of time self-heats the bimetallic member causing it to bend and move a motion transfer connecting plate and concomitantly a latch to unlatch a member from a spring biased bell crank mechanism allowing the bell crank mechanism to rotate and thereby move the contacts to the open circuit position in a conventional manner. According to the invention, an arc sensing circuit is included in the circuit breaker and is electrically connected to a supplemental actuator for unlatching the bell crank mechanism to move the contacts to the open circuit position. In one preferred embodiment, the actuator includes a solenoid which, when energized by the arc sensing circuit, transfers motion to a lever mounted in a position to be pushed by the solenoid upon energization thereof with the lever coupled to an extension of the connecting plate for pulling the connector plate along with the latch to unlatch the bell crank mechanism. In another preferred embodiment in which the actuator includes a solenoid, an arm is attached to the latch causing the latch to pivot and unlatch the bell crank mechanism. In still another preferred embodiment, an elongated piezoresistive member is coupled to the connecting plate and is electrically energizable upon energization of the arc sensing circuit. Upon energization, the piezoresistive member bends to pull the connecting plate along with the latch to unlatch the bell crank mechanism as in the previously described embodiment. According to a feature of the invention, a suitable indicating means, such as a visual indicator, may be provided which is energized when the arc sensing circuit causes the circuit breaker to trip to distinguish the trip from one caused by a current overload. In an alternative preferred embodiment, a mechanical relay having a coil is included in the circuit breaker in which the coil is energized upon the occurrence of an overload current causing the relay to open. Upon opening of the relay, the current is temporarily shunted through a transistor to eliminate arcing of the mechanical contacts. Once the mechanical contacts have opened sufficiently far to avoid arcing, the transistor is then de-energized along with load current.